The benefit of the filing and priority dates of the International and Japanese Applications is respectfully requested.
1. Field of the Invention
The present invention relates to a WDM (Wavelength Division Multiplexing) optical communication system suitable for WDM optical communications utilizing a plurality of signals included in a 1.58-xcexcm wavelength band (1570 nm to 1620 nm).
2. Related Background Art
WDM optical communications are a communication technique enabling large-capacity optical communications by utilizing a plurality of signals having wavelengths different from each other. For the WDM optical communications, light in a 1.55-xcexcm wavelength band (1530 nm to 1560 nm) is utilized since the transmission loss of silica-based optical fibers which have been widely utilized as transmission lines is small in the 1.55-xcexcm wavelength band, and since the gain of Er-doped optical fiber amplifier (EDFA: Er-Doped Fiber Amplifier) for amplifying signals is high in the 1.55-xcexcm wavelength band.
Examples of transmission lines applicable to WDM optical communications in the 1.55-xcexcm wavelength band include a single-mode optical fiber having a zero-dispersion wavelength in a 1.3-xcexcm wavelength band (1260 nm to 1350 nm), a dispersion-shifted optical fiber having a zero-dispersion wavelength in the 1.55-xcexcm wavelength band, and a hybrid transmission line in which they are mixed. Since the single-mode optical fiber has a large positive dispersion in the 1.55-xcexcm wavelength band, the single-mode optical fiber and a dispersion-compensating optical fiber (dispersion compensator) having a large negative dispersion in the 1.55-xcexcm wavelength band are often combined together, so as to compensate for dispersion in the 1.55-xcexcm wavelength band.
On the other hand, it has been known that the dispersion-shifted optical fiber, in general, is likely to deteriorate the waveform of each signal due to nonlinear optical phenomena, such as four-wave mixing in particular, in optical communications in the 1.55-xcexcm wavelength band since it has a very small absolute value of dispersion (nearly zero) in the 1.55-xcexcm wavelength band and a small effective area. Since such a waveform deterioration caused by nonlinear phenomena cannot be restored, the occurrence of nonlinear optical phenomena must be suppressed to a minimum. For suppressing the occurrence of nonlinear optical phenomena, the power of each signal may be lowered. In the case of long-distance optical communications, however, if the signal power is lowered, then the intervals between repeaters must be shortened, whereby the cost rises along with the increase in optical amplifiers and the like which are disposed. Hence, as another effective method of suppressing nonlinear optical phenomena, optical communications may be carried out in a wavelength band, other than the 1.55-xcexcm wavelength band, in which the absolute value of dispersion is sufficiently high to maintain effects of non-linear dispersion within tolerable limit.
On the other hand, larger capacities are demanded in optical communications. From this viewpoint, the research and development aimed at enlarging the amplification bandwidth by use of optical fiber amplifiers has been under way. Also, the research and development of optical fiber amplifiers capable of amplification in wavelength bands other than the 1.55-xcexcm wavelength band has been under way, and an optical fiber amplifier which can amplify signals in the 1.58-xcexcm wavelength band, for example, has been realized.
From the foregoing technical background, WDM optical communications utilizing a plurality of signals included in the 1.58-xcexcm wavelength band in place of or in addition to the 1.55-xcexcm wavelength band have been taken into consideration. The transmission loss of silica-based optical fibers is relatively small even in the 1.58-xcexcm wavelength band, so that there are no inconveniences in terms of transmission loss.
As configurations of WDM optical communication system which transmit signals in the 1.58-xcexcm wavelength band, those described in a literaturexe2x80x94A. K. Srivastava et al., ECOC""98, postdeadline paper, pp. 73-75 (1998)xe2x80x94, a literaturexe2x80x94Y. Yano, et al., ECOC""98, pp. 261-262 (1998)xe2x80x94, a literature xe2x80x94T. Sakamoto, et al., OAA""98, TuB3, pp. 88-91 (1998)xe2x80x94, and a literaturexe2x80x94M. Jinno, et al., IEEE Photon. Technol. Lett., Vol. 10, No. 3, pp. 454-456 (1998)xe2x80x94, for example, have been known. Each of the transmission lines of WDM optical communication systems described in these literatures is constituted by a dispersion-shifted optical fiber alone.
The inventors have studied conventional WDM optical communication systems and, as a result, have found a problem as follows. In the dispersion-shifted optical fiber having a zero-dispersion wavelength in the 1 .55-xcexcm wavelength band, the absolute value of dispersion in the 1.58-xcexcm wavelength band is about 2 to 3 ps/nm/km, whereby four-wave mixing is relatively hard to occur. Employing such a dispersion-shifted optical fiber in a transmission line can increase the power of each signal, thereby making it possible to elongate repeater intervals. If each of the signals have a higher power while the number of signals (number of channels) subjected to wavelength multiplexing increases, however, then cross-phase modulation (XPM), which is another nonlinear optical phenomenon, becomes remarkable. In addition, for-wave mixing has also been in a serious problem, if a channel spacing of signals becomes lower or signal input power becomes much higher.
In order to overcome problems such as the one mentioned above, it is an object of the present invention to provide a WDM optical communication system which: effectively suppresses the waveform deterioration resulting from nonlinear optical phenomena, such as four-wave mixing and cross-phase modulation in particular, of each signal in the 1.58-xcexcm wavelength band in a transmission line including a dispersion-shifted optical fiber having a zero-dispersion wavelength in the 1.55-xcexcm wavelength band.
The WDM optical communication system according to the present invention is a WDM (Wavelength Division Multiplexing) optical communication system for transmitting a plurality of signals included in the 1.58-xcexcm wavelength band (1570 nm to 1620 nm). This WDM optical communication system comprises at least one hybrid transmission unit for transmitting the plurality of signals. This hybrid transmission unit comprises at least a single-mode optical fiber and a dispersion-shifted optical fiber, whereas these optical fibers are arranged such that signals emitted from an optical transmitter successively pass though the single-mode optical fiber and the dispersion-shifted optical fiber. For enabling bidirectional communications of signal, the hybrid transmission unit may comprise a dispersion-shifted optical fiber and two single-mode optical fibers disposed so as to sandwich the dispersion-shifted optical fiber therebetween. Namely, the hybrid transmission unit in the WDM optical communication system according to the present invention is configured such that signals pass through a single-mode optical fiber before entering the dispersion-shifted optical fiber, regardless of the traveling direction of signal.
The single-mode optical fiber has a zero-dispersion wavelength in the 1.3-xcexcm wavelength band. (1260 nm to 1350 nm) and an effective area ASMF at a wavelength of 1.58 xcexcm. The dispersion-shifted optical fiber has a zero-dispersion wavelength in the 1.55-xcexcm wavelength band (1530 nm to 1565 nm). If the zero-dispersion wavelength of the dispersion-shifted optical fiber is set to the 1.55-xcexcm wavelength band, then the accumulated part of dispersion in this wavelength band can be made substantially zero. Preferably, the dispersion-shifted optical fiber has a dispersion with an absolute value of 0.5 ps/nm/km or more at a wavelength of 1.58 xcexcm. It is because of the fact that dispersion is intentionally generated to a certain extent in the signal wavelength band, so as to reduce the influence of four-wave mixing, thereby enabling high-density wavelength multiplexing. Preferably, the upper limit of dispersion at a wavelength of 1.58 xcexcm is 5 ps/nm/km. This is for keeping waveform from deteriorating due to the increase in cumulative dispersion. Preferably, at a wavelength of 1.58 xcexcm, the single-mode optical fiber has an effective area ASMF greater than the effective area ADSF of the dispersion-shifted optical fiber. As the single-mode optical fiber, not only a common type doped with GeO2, but also pure silica core type optical fibers can be employed.
In this WDM optical communication system, as in the foregoing, a plurality of signals included in the 1.55-xcexcm wavelength band would propagate through a dispersion-shifted optical fiber after propagating through the single-mode optical fiber whether the hybrid transmission unit has a configuration for unidirectional or bidirectional optical communications. Therefore, the peak power of each signal at the output end of the single-mode optical fiber is kept low because of its attention. Namely, the single-mode optical fiber disposed upstream from the dispersion-shifted optical fiber in the traveling direction of signal functions as an attenuator for lowering the incident light power to the dispersion-shifted optical fiber beforehand. Also, since the single-mode optical fiber has a relatively large effective area ASMF and a high dispersion DSMF, not only four-wave mixing but also cross-phase modulation is restrained from occurring. As a consequence, the influence of interactions between wavelengths is lowered, whereby the widening of spectrum and deterioration of waveform in each signal at the output end of the single-mode optical fiber are sufficiently suppressed. Also, since each signal fed into the dispersion-shifted optical fiber has already propagated through the single-mode optical fiber, it has a lower peak power. Therefore, nonlinear optical phenomena are harder to occur in the dispersion-shifted optical fiber as well, whereby the waveform deterioration of each signal at the output end of the dispersion-shifted optical fiber can be kept low.
On the other hand, the single-mode optical fiber preferably has a fiber length longer than its effective length Leff (unit being km) by 5 km or more (e.g., by 10 km or more). Namely, since the waveform deterioration due to nonlinear optical phenomena is not considered to occur substantially in the surplus part of single-mode optical fiber, the power of each signal entering the dispersion-shifted optical fiber can effectively be lowered while elongating its transmission distance. In the case where the fiber length of single-mode optical fiber is longer than the effective length Leff as such, it is preferred that an optical amplifier be disposed on the exit end side of the dispersion-shifted optical fiber so as to compensate for the loss in signals emitted from the dispersion-shifted optical fiber (in order to keep the incident light power to the dispersion-shifted optical fiber from increasing). In the case where the hybrid transmission unit has a configuration capable of bidirectional optical communications in which the dispersion-shifted optical fiber is sandwiched between two single-mode optical fibers, it is also preferred that an optical amplifier be installed on the exit end side of the dispersion-shifted optical fiber in the traveling direction of signal in view of the fiber length of single-mode optical fibers.
Letting D be the dispersion at a wavelength of 1.58 xcexcm, P be the optical power per channel in a plurality of signals launches from the single-mode optical fiber to the dispersion-shifted optical fiber, and CS be the individual channel spacing in the plurality of signals, the dispersion-shifted optical fiber satisfies the condition of       P          D      ·      CS        ≤      4.2    xc3x97                            10          13                ⁡                  [                                    (                              W                ·                m                            )                        /            s                    ]                    .      
If the dispersion-shifted optical fiber satisfies such a condition with respect to the single-mode optical fiber, then the transmission line length can be elongated without enlarging the wavelength band in which dispersion becomes greater. More preferably, the dispersion-shifted optical fiber satisfies the condition of       P          D      ·      CS        ≤      2.6    xc3x97                            10          13                ⁡                  [                                    (                              W                ·                m                            )                        /            s                    ]                    .      
If this condition is satisfied, then the usable signal wavelength band enlarges, and the transmission line length can be elongated without dispersion compensation even in the case where the number (number of channels) of signals increases.
The WDM communication system according to the present invention may further comprise an optical amplifier, disposed at least on the entrance end side of the single-mode optical fiber on which signals are incident, for amplifying the signals. In this configuration, the single-mode optical fiber preferably has a fiber length of 10 km or more. It is because of the fact that, if the single-mode optical fiber has a length of 10 km or more, then the peak power of each signal entering the dispersion-shifted optical fiber can be kept low even when the power of each signal reaching the single-mode optical fiber .from the optical amplifier is increased, which is effective in suppressing the occurrence of nonlinear optical phenomena in the dispersion-shifted optical fiber. Thus, not only the waveform deterioration of each signal passed through the hybrid transmission unit constituting a part of the WDM optical communication system is suppressed, but also the output power of optical amplifier can be increased, by which the cost of the whole system can be cut down.
The hybrid transmission unit in the WDM optical communication system according to the present invention may further comprise a dispersion-compensating optical fiber (dispersion compensator) having a dispersion with an opposite sign against the dispersion of the single-mode optical fiber in the 1.58-xcexcm wavelength band. In this case, the dispersion of the single-mode optical fiber is compensated for by the dispersion-compensating optical fiber, whereby the pulse waveform of each signal is restored.
There are various modes concerning the arrangement of the dispersion-compensating optical fiber. For example, while the dispersion-shifted optical fiber is divided into two or more components, the dispersion-compensating optical fiber can be arranged between a set of components selected therefrom. Also, an optical amplifier for amplifying the signals to enter the single-mode optical fiber may be disposed on the entrance end side of the single-mode optical fiber, whereas the dispersion-compensating optical fiber may be arranged so as to sandwich the optical amplifier with the single-mode optical fiber. The dispersion-compensating optical fiber is characterized in that it has a very high nonlinear characteristic and a very large dispersion, whereby the upper limit of power in signals which can propagate therethrough is low. Since the dispersion-compensating optical fiber is disposed upstream from the optical amplifier in the traveling direction of signal, the signals before being amplified inevitably propagate through the dispersion-shifted optical fiber in the WDM optical communication system according to the present invention. Due to this configuration, nonlinear optical phenomena are restrained from occurring in the dispersion-compensating optical fiber, whereby the waveform of each signal can be kept from deteriorating.
Preferably, whether the hybrid transmission unit has a configuration for unidirectional or bidirectional optical communications in the WDM optical communication system according to the present invention, the single-mode optical fiber has a polarization mode dispersion of 2 psxc2x7kmxe2x88x92xc2xd or less at a wavelength of 1.58 xcexcm, and the dispersion-shifted optical fiber has a polarization mode dispersion of 2 psxc2x7kmxe2x88x92xc2xd or less at a wavelength of 1.58 xcexcm. In the case of the configuration in which the hybrid transmission unit is provided with a dispersion-compensating optical fiber, the dispersion-compensating optical fiber preferably has a polarization mode dispersion of 2 psxc2x7kmxe2x88x92xc2xd or less at a wavelength of 1.58 xcexcm. Preferably, letting B be the bit rate of signals, the,hybrid transmission unit as a whole has a cumulative polarization dispersion of: 1/(4B) or less at a wavelength of 1.58 xcexcm in the WDM optical communication system according to the present invention. In each of these cases, the occurrence of cross-phase modulation depending on the polarization state of signals and the occurrence of waveform deterioration resulting from dispersion can be suppressed more effectively.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.